The present invention relates to a method for determining characteristic variables for electrical states of an electrochemical energy storage battery. The present invention also relates to a monitoring device for an electrochemical energy storage battery having an evaluation unit and a measurement unit for measurement of the battery terminal voltage, the battery terminal current, and the battery temperature.
In certain applications, there is a need to determine or predict the instantaneous state of an electrochemical energy storage battery, such as the state of charge or the capability to be loaded with a high current. By way of example, the capability of a starter battery to start a motor vehicle with an internal combustion engine is governed by the state of charge, the state of aging, and the apparent capacity loss of the battery, since the current level which can be drawn from the starter battery and its power output are limited. It may be important to determine the state of charge and the starting capability of a starter battery in situations in which, for example, the engine is being operated intermittently, since the vehicle power supply system together with its loads is still operated during the periods when the engine is stopped, even though the generator is not producing any current. The monitoring of the state of charge and of the starting capability of the energy storage battery must in situations such as these ensure that the energy content of the energy storage battery always remains sufficient to start the engine.
Widely differing methods are known for measurement of the state of charge and for determination of the load behavior of energy storage batteries. For example, integrating instruments (e.g., amp-hour (Ah) counters) are used, with the charging current possibly being taken into account and weighted with a fixed charging factor. Since the usable capacity of an energy storage battery is highly dependent on the magnitude of the discharge current and on the temperature, methods such as these also do not allow any satisfactory statement to be made about the usable capacity which can still be drawn from the battery.
By way of example, it is known from DE 22 42 510 C1 to provide a method for measuring the state of charge in which the charging current is weighted with a factor which is itself dependent on the temperature and on the state of charge of the battery.
DE 40 07 883 A1 describes a method in which the starting capability of an energy storage battery is determined by measuring the battery terminal voltage and the battery temperature, and comparing with a state of charge family of characteristics which is applicable to the battery type to be tested.
DE 195 43 874 A1 discloses a calculation method for the discharge characteristic and remaining capacity measurement of an energy storage battery in which the current, voltage, and temperature are likewise measured, and in which the discharge characteristic is approximated by a mathematical function with a curved surface.
DE 39 01 680 C1 describes a method for monitoring the cold starting capability of a starter battery in which the starter battery is loaded with a resistance at times. The voltage dropped across the resistance is measured and is compared with empirical values to determine whether the cold starting capability of the starter battery is still sufficient. In this case, the starter battery is loaded by the starting process.
Furthermore, DE 43 39 568 A1 discloses a method for determining the state of charge of a motor vehicle starter battery in which the battery current and rest voltage are measured, and the state of charge is deduced from them. In this case, the battery temperature is also taken into account. The charging currents measured during different time periods are compared with one another, and a remaining capacity is determined from them.
DE 198 47 648 A1 describes a method for learning a relationship between the rest voltage and the state of charge of an energy storage battery for the purpose of estimating the storage capability. A measure for the electrolyte capacity of the electrolyte in the energy storage battery is determined from the relationship between the rest voltage difference and the amount of current drawn during the load phase. This makes use of the fact that the rest voltage rises approximately linearly with the state of charge in the higher state of charge ranges which are relevant in practice.
One problem in determining the state of an electrochemical energy storage battery using the already known methods is that wear occurs not only when rechargeable energy storage batteries are being discharged and charged but also when they are stored without any load being applied, and the relevant wear factors are not all considered.
In the case of a lead-acid rechargeable battery, the electrolyte is composed of dilute sulfuric acid, that is to say, a solution of sulfuric acid in water. In the completely charged state, this is typically an approximately 4 to 5 molar solution. During the discharge reaction, sulfuric acid in the electrolyte is consumed at both electrodes on the basis of the reaction equation:Positive electrode: PbO2+H2SO4+2H++2e−→PbSO4+2H2ONegative electrode: Pb+H2SO4→Pb+2H++2e−and, furthermore, H2O is formed at a positive electrode. In consequence, the concentration and the relative density of the electrolyte fall during discharging, while they rise again during the charging reaction, which takes place in the opposite sense.
If the sulfuric acid which is formed during the charging reaction has the capability for convection in the earth's field of gravity, then it has the tendency to fall in layers to the bottom of the cell vessel for the lead-acid rechargeable battery cells. The electrolyte in the lower area of the respective cell vessel thus has a higher concentration than that in the upper area of the cell vessel. In the case of a lead-acid rechargeable battery, this state is referred to as acid stratification.
Since both the charging and discharge reaction as well as the parasitic reactions, such as gas development, corrosion etc., are in general influenced by the electrolyte concentration, acid stratification leads to the cell state not being uniform.
It would be advantageous to provide an improved method for determining characteristic variables for electrical states of an energy storage battery. It would also be advantageous to provide an improved monitoring device.